dist: scylla_util.py: get_mode_cpuset(): don't let false alarm error messages
dist: scylla_util.py: get_mode_cpuset(): don't let false alarm error messages

Don't let perftune.py print false alarm error message when we calculate
a compute CPU set for tuning modes.

This may happen when we calculate a CPU set for non-MQ tuning modes on
small systems on which these modes are forbidden because they would
result in a zero CPU set, e.g. sq_split on a system with a single
physical core.

We are going to utilize a newly introduced --get-cpu-mask-quiet execution
mode introduced to the seastar/script/perftune.py by the
"perftune.py: introduce --get-cpu-mask-quiet" series which would return
a zero CPU set if that's what it turns out to be instead of exiting with
an error what --get-cpu-mask would do in such a case.

The rest of scylla_util.py logic is going to handle a zero CPU set
returned by get_mode_cpuset() correctly.

Fixes #4211
Fixes #4443

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@scylladb.com>
Message-Id: <20190731212901.9510-1-976c0f8d0852ed6439ad273880b400a75a7c41d0@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla